The field of the invention is nuclear magnetic resonance imaging methods and systems. More particularly, the invention relates to intra-operative MR imaging in which a medical instrument is guided by acquiring realtime images.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.2, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated, this signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received NMR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
Intra-operative MR imaging is employed during a medical procedure to assist the doctor in guiding an instrument. For example, during a needle biopsy the MRI system is operated in a realtime mode in which image frames are produced at a high rate so that the doctor can monitor the location of the needle as it is inserted. The needles can cause artifacts on the images that allow the clinician to observe their location. However, in some instances the image prescriptions and the needle material may combine to render the artifact so small that the needle cannot be easily detected or so large that the needle cannot be localized.
A locator device such as that sold commercially by Image Guided Technologies, Inc. under the trademark "Flashpoint" and described in U.S. Pat. Nos. 5,622,170 and 5,617,857 may be used to track the location of the instrument and provide coordinate values to the MRI system which enable it to mark the location of the instrument in each reconstructed image. The medical instrument is attached to a handpiece that is manipulated by the physician and whose position is detected by surrounding sensors. For example, the handpiece may emit light from two or more light emitting diodes which is sensed by three stationary cameras.
Because the coordinate system of the locator device is separate from the coordinate system of the MRI system, the location of the instrument as measured by the locator device must be transformed to the MRI system coordinates to accurately indicate the instrument location in the reconstructed image. This transformation is accomplished using a transformation matrix which is created off-line using a calibration procedure. The calibration procedure is an elaborate and time consuming process which is performed periodically by maintenance personnel. If the calibration is inaccurate, the location of the instrument in the image displayed to the doctor is inaccurate. This is unacceptable in procedures that rely on an accurate placement of the instrument in the imaged patient. Furthermore, it is possible to select from a range of handpieces with a range of needle lengths. A simple method of verifying the system accuracy and correct selection of handpieces and needle lengths would increase the clinician confidence in the system and possibly prevent adverse events.